1. Field of the Art
The present invention relates generally to a dot-matrix printer capable of printing characters such as letters, symbols and graphical representations, with a matrix of dots. More particularly, the invention is concerned with such a dot matrix printer which is free from printing troubles due to a drop in drive voltage of a print head, or an abnormal rise in temperature of the print head, when dots are formed continuously with a relatively high density.
2. Related Art Statement
In a common dot-matrix printer, print elements provided on a print head are disposed in a row and selectively activated to form dots in a vertical row of a dot matrix pattern corresponding to a character. It is ideal to determine a capacity of a power supply to the print head so that a drive voltage will not significantly drop even if all print elements are activated to form the corresponding all dots in successive rows continuously. Actually, an average power requirement for printing ordinary letters and symbols is only a half or less than a half of that for full-dot printing with concurrent activation of all print elements. For this reason, dot-matrix printers usually use a power supply of relatively small capacity that meets the actual average power consumption, for reduced cost of the power supply.
With such a relatively small capacity power supply, however, some of the graphical representations or figures available on the printer may not be correctly printed. More specifically, the printing of graphical representations more or less requires a comparatively large number of dots to be formed in a vertical row of a dot matrix pattern, that is, requires a higher dot ratio which is a ratio of the number of the print elements that are activated, with respect to the number of the print elements provided on the print head, than the printing of ordinary letters or symbols. Accordingly, the printing of some graphical representations requires a relatively larger energy input, which may lead to a shortage of power stored in a capacitor used as a power source for the printer. That is, the voltage level of the capacitor may drop and become insufficient to drive all of the intended print elements. Thus, some of the print elements may fail to be operated as required, if the printer is driven with a relatively small power supply. In such events, the corresponding dots will not be formed with a sufficient density, resulting an a poor print quality.
A control arrangement for a dot-matrix printer to solve the above-addressed problem is proposed in Japanese Patent Application which was laid open in 1984 under Publication No. 59-16765. The proposed control arrangement is adapted to obtain a "dot ratio" which is a ratio of the number of dots which are to be formed in each print line (each line of characters), with respect to the maximum number of dots which are formable in each print line. In the event that the obtained dot ratio of a given print line exceeds a predetermined reference value corresponding to a capacity of a power source for the print head of the printer, the characters in that print line are printed in plural movements of the print head along the print line. For example, some of the print elements that should be activated to form dots in each vertical row of a matrix pattern are activated while the print head is moved in one direction along the print line, and the remaining print elements that should be activated are activated while the print head is moved in the opposite direction. On the other hand, if the obtained dot ratio is less than the reference value, that print line is printed in one movement of the print head.
In the above proposed control arrangement, the obtained dot ratio of a print line is an average ratio of the dots which will be formed in the print line, with respect to the maximum dot number of the line. It will be understood that the average dot ratio of a print line may be lower than the predetermined reference value even if that print line contains a portion whose dot ratio is very high. For example, the average dot ratio of a print line as a whole may be comparatively low even if that line contains a succession of graphic representations which are formed by dots of a very high density per unit area. In this instance, the dot ratio of the graphic portion of the print line is far higher than the remaining portion which consists mainly of letters. However, the graphic portion is printed in one movement of the print head. Consequently, when the graphic portion is printed continuously, the drive voltage of the print head will drop, causing the print elements to fail to operate as required. Thus, the graphic portion may not be printed with a satisfactory print quality due to omission of some dots.
In the art of dot-matrix printing, it is also recognized that an abnormal temperature rise or overheat of a print head will result in poor printing quality. More specifically, in the case of a wire-dot printer, an insulating property of insulators for solenoids to activate the print wires will be reduced when the temperature of the print head is excessively elevated. Consequently, the solenoids tend to fail to be energized as required, whereby the corresponding dots may not be formed. In the case of a thermal-matrix printer, an excessive rise of the print head temperature will cause the thermal ribbon to be more or less fused by portions of the print head other than the heat-generating elements which are activated. In this case, a print sheet or other recording medium is soiled with an ink, or the characters may not be printed clearly.
To solve the above problem, it has been attempted to use a sensor to detect the temperature of the print head, and dissipate heat from the print head with a suitable method, when the detected temperature exceeds a predetermined upper limit. For example, the heat dissipation is accomplished by printing a line of characters in plural movements of the print head, or by interrupting a printing cycle part way through a print line or at the end of the line for a necessary length of time. Another solution to the same problem is proposed, wherein the heat dissipation is effected when the average dot ratio of a print line exceeds a predetermined reference value, as previously described.
The above proposed solutions suffer the following inconveniences. Namely, the use of a temperature sensor will complicate the printer and increase the cost. In the case of cooling the print head when the dot ratio has exceeded the reference value, the dot ratio does not necessarily accurately represents the temperature of the print head, because the heat is dissipated from the print head while the print sheet is fed or the printing cycle is interrupted. Accordingly, the heat dissipation may be insufficient, or the printer spend an unnecessarily long time for heat dissipation from the printer. In the former case, the problem of heat will not be completely eliminated. In the latter case, the printing efficiency is reduced.